1. Field of the Invention
The present invention relates to touch panels and display devices.
2. Description of the Related Art
Conventionally, touch panels mounted on display devices such as LCDs (Liquid Crystal Displays) have been transparent touch panels that input control signals through operations with a pen or a finger in accordance with displayed information. Among those transparent touch panels, there are SAW touch panels. In a SAW touch panel, surface acoustic wave that is propagated on a glass surface is utilized as a means of detecting touched locations.
Japanese Unexamined Patent Document 2002-222041 (hereinafter referred to as Patent Document 1) discloses the conventional SAW touch panel. The touch panel device disclosed in Patent Document 1 is formed with a glass substrate, and a touched location is detected by spotting an interrupted location of the SAW being propagated on the surface of the glass substrate. Accordingly, the most advantageous features of this touch panel device are that the light transmittance is high and that the screen of the display device is not damaged. Also, as the operation area is a solid glass surface, scratches are not easily made. Accordingly, the durability of this touch panel device is higher than that of a touch panel device of any other type.
However, as the touch panel device disclosed in Patent Document 1 is formed with only one glass plate, the shock resistance is poor. Particularly, in a case where the glass thickness is reduced to produce a thin device, the glass substrate can be easily broken even by small impact. Also, when a drop of water or oil adheres to the glass surface, the propagation of SAW is interrupted, resulting in an erroneous input.
So as to solve the above problems, a touch panel device disclosed in Japanese Patent Publication No. 3010669 (hereinafter referred to as Patent Document 2) has been developed. The touch panel device disclosed in Patent Document 2 has a transparent film placed over a glass substrate, with spacers being interposed in between. In this structure, the transparent substrate on which a surface acoustic wave is being propagated is unexposed. Thus, a scratch or a drop of water cannot adversely affect the SAW propagation characteristics.
In the touch panel device disclosed in Patent Document 2, the transparent film is placed over the glass substrate on which SAW is being propagated, with the spacers being interposed between the transparent film and the glass substrate. However, in a case where the touch panel device is employed in a compact, thin mobile device, the energy of propagated SAW is so small that a large propagation loss is caused at the spacers formed on the glass substrate. As a result, accurate location detection cannot be carried out.